Scientists abuzz over cosmic symphony in 15 years of exotic star data

The worlds of astro- and theoretical physics are humming with excitement over data suggesting that gravitational waves are rippling through spacetime and buffeting the Earth.

Following a weeklong hype campaign on social media, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) announced the discovery together with the publication of several papers.

The articles were published in the The Astrophysical Journal Letters and are open-access.

The scientists used exotic stars called pulsars as detectors by measuring slight changes in the timing of their radio wave beams as they swept across the Earth.

They collected data from 68 pulsars scattered across half the Milky Way galaxy to form a detector called a pulsar timing array.

In a statement announcing the discovery, they explain that a pulsar is the ultra-dense remnant of a massive star’s core following its demise in a supernova.

Pulsars spin rapidly, sweeping beams of radio waves through space so that they appear to pulse when seen from the Earth.

The fastest of these objects, called millisecond pulsars, spin hundreds of times each second. Their pulses are very stable, making them useful as precise cosmic timepieces.

“Pulsars are actually very faint radio sources, so we require thousands of hours a year on the world’s largest telescopes to carry out this experiment,” said NanoGrav co-director Maura McLaughlin, from West Virginia University.

Einstein’s theory of general relativity predicts precisely how gravitational waves should affect pulsar signals.

By stretching and squeezing the fabric of space, gravitational waves affect the timing of each pulse in a small but predictable way, delaying some while advancing others.

These shifts are correlated for all pairs of pulsars in a way that depends on how far apart the two stars appear in the sky.

“The large number of pulsars used in the NANOGrav analysis has enabled us to see what we think are the first signs of the correlation pattern predicted by general relativity,” said NANOGrav’s other co-director Xavier Siemens, from Oregon State University.

NANOGrav is a collaboration of over 190 scientists from the US and Canada who use pulsars to search for gravitational waves. International collaborations using telescopes in Europe, India, Australia and China have independently reported similar results.

NANOGrav is part of the International Pulsar Timing Array, which comprises several pulsar timing array consortiums around the world.

These include the European Pulsar Timing Array, CSIRO’s Australia Telescope National Facility, the Indian Pulsar Timing Array, and the African Pulsar Timing Array.

The Africa Pulsar Timing Array uses the Square Kilometre Array precursor telescope MeerKAT, located 90km outside Carnarvon in the Northern Cape.

Although data from MeerKAT was not part of today’s announcement, the telescope will collaborate on future research.

The MeerKAT telescope in the Karoo, 90km outside Carnarvon. Photo: SARAO

Black holes and cosmic strings

Regarding the implications of NANOGrav’s discovery, Oxford University astronomy professor Chris Lintott explained that the gravitational waves are believed to most likely be the result of the merging of supermassive black holes at the centres of galaxies.

Lintott said that detecting the small variations in pulsar timings was a staggeringly difficult measurement.

“Requiring, amongst much else, knowing the position of the centre of mass of the solar system to a precision of less than 100m,” he said.

“Previous results were confused because we didn’t know where Jupiter was accurately enough.”

Lintott explained there was slight controversy around today’s announcement, as the headline statistical test shows a one in a thousand chance that the observed pattern of delays is a coincidence.

“In the parlance of stats, it’s a 4-sigma detection. There’s a convention in particle physics that 5-sigma is required for a ‘discovery’. I’m told there is some controversy within the team as to what should be claimed,” he said.

“So, what’s causing the Earth to shake with these gravitational waves? The results are consistent with waves from supermassive black hole binaries — in orbit, but not quite merging with each other, as these behemoths whirl, they lose energy by causing space itself to ripple,” said Lintott.

“Imagine two black holes, each the mass of a few million suns, locked in a 15-year orbit, slowly spiralling together, and you’ve got the right picture.”

Scientific American reports that another interpretation of the results sees the gravitational waves originating from a vibrating network of high-energy cosmic strings.

These could provide scientists with detailed information about the fundamentals of physical reality.

A third hypothesis suggests that scientists may have spotted the creation of many small black holes at the dawn of time, pointing to a possible explanation for dark matter.

NANOGrav team member and University of Connecticut astrophysicist Chiara Mingarelli acknowledged that they are not sure what is generating this signal — but added that many people are excited about the discovery.

Lintott believes the astronomy community need not yet reach for exotic explanations for the gravitational waves.

“At least one of the papers out today presents alternative exotic theoretical explanations,” he said.

“I suspect this will get a lot of attention, but I don’t see the need to reach for them yet. We’d expect a contribution from black hole binaries — that’s likely what we’ve seen.”

Illustration by Olena Shmahalo via NANOGrav.

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Scientists abuzz over cosmic symphony in 15 years of exotic star data